With the recent rapid progress for reduction in size and weight of electronic equipments such as mobile phones and personal computers, there is an increasing demand for secondary batteries having a high energy density as a power source for driving these electronic equipments. Under these circumstances, the batteries having a large charge/discharge capacity per unit weight and unit volume and high repeated charge/discharge cycle characteristics have been recently noticed.
Hitherto, as one of positive electrode active substance particles useful for high energy-type lithium ion secondary batteries, there is known lithium nickelate LiNiO2 of a layer (rock salt type) structure having a 4 V-order voltage. The LiNiO2 particles are inexpensive and excellent in output characteristics as compared to lithium cobaltate LiCoO2 particles as generally used positive electrode active substance particles, and therefore have been mainly applied to a power source for power tools. In recent years, the LiNiO2 particles also tend to be now applied to a driving power source for electric vehicles in view of their characteristics. However, the lithium ion secondary batteries using the aforementioned LiNiO2 particles as active substance particles therefor have problems such as deterioration in repeated charge/discharge cycle characteristics at a high voltage and generation of gases from the batteries upon storage under high-temperature conditions owing to elution of constitutional ions other than Li+ ion from the active substance particles or incompleteness of the reaction between raw material particles upon synthesis thereof. For these reasons, it has been demanded to further improve powder characteristics of the active substance particles.
It is well known in the art that in the NiO6 octahedron constituting the lithium nickelate-based positive electrode active substance particles, the Ni3+ ion is kept in a low spin state at room temperature, in other words a d-orbital electronic configuration of the Ni3+ ion is represented by t2g6eg1. Therefore, the conventional lithium nickelate-based positive electrode active substance particles have failed to have such a high stability. In addition, the Ni2+ ion has an ionic radius close to Li+ ion, and therefor tends to suffer from structural defects such as cation mixing upon synthesis of the positive electrode active substance. For this reason, it has been contemplated that the Ni3+ ion in the conventional lithium nickelate-based positive electrode active substance particles is substituted with Co3+ ion or Al3+ ion to improve characteristic thereof (Non-Patent Literature 1).
On the other hand, even the lithium nickelate composite oxide particles whose Ni3+ ion is substituted with ions of different kinds of elements still comprise a more than necessary amount of lithium carbonate or lithium hydroxide as an impurity phase. These unreacted lithium compounds are main factors causing increase in a powder pH value of the lithium nickelate composite oxide particles, and tend to induce not only gelation of an electrode slurry upon production of the electrode slurry, but also generation of gases from the batteries upon storage under high-temperature conditions when the resulting secondary battery is subjected to charging and discharging cycles. In particular, in order to avoid remarkable adverse influence of the lithium hydroxide, the unreacted substance being present on a surface of the respective particles is carbonated (Patent Literatures 1 and 2), or removed by washing with water and drying (Non-Patent Literature 2).
In order to further improve the lithium nickelate composite oxide particles, there has been proposed the method of subjecting the particles as core particles to surface treatment in which the unreacted lithium carbonate or lithium hydroxide is converted into the other lithium compound. The coating film obtained by the surface treatment acts as a protective film against hydrofluoric acid produced by charging and discharging cycles of the resulting battery to prolong a service life of the battery (Non-Patent Literature 3).